/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.util;

import java.util.Comparator;
import org.apache.hadoop.io.IntWritable;

/**
 * An implementation of the core algorithm of MergeSort. <br/>
 * 归并排序的一个实例，因为之前对归并排序有过了解，不深入解读了。
 */
public class MergeSort {
	// Reusable IntWritables
	IntWritable I = new IntWritable(0);
	IntWritable J = new IntWritable(0);

	// the comparator that the algo should use
	private Comparator<IntWritable> comparator;

	public MergeSort(Comparator<IntWritable> comparator) {
		this.comparator = comparator;
	}

	public void mergeSort(int src[], int dest[], int low, int high) {
		int length = high - low;

		// Insertion sort on smallest arrays
		if (length < 7) {
			for (int i = low; i < high; i++) {
				for (int j = i; j > low; j--) {
					I.set(dest[j - 1]);
					J.set(dest[j]);
					if (comparator.compare(I, J) > 0)
						swap(dest, j, j - 1);
				}
			}
			return;
		}

		// Recursively sort halves of dest into src
		int mid = (low + high) >>> 1;
		mergeSort(dest, src, low, mid);
		mergeSort(dest, src, mid, high);

		I.set(src[mid - 1]);
		J.set(src[mid]);
		// If list is already sorted, just copy from src to dest. This is an
		// optimization that results in faster sorts for nearly ordered lists.
		if (comparator.compare(I, J) <= 0) {
			System.arraycopy(src, low, dest, low, length);
			return;
		}

		// Merge sorted halves (now in src) into dest
		for (int i = low, p = low, q = mid; i < high; i++) {
			if (q < high && p < mid) {
				I.set(src[p]);
				J.set(src[q]);
			}
			if (q >= high || p < mid && comparator.compare(I, J) <= 0)
				dest[i] = src[p++];
			else
				dest[i] = src[q++];
		}
	}

	private void swap(int x[], int a, int b) {
		int t = x[a];
		x[a] = x[b];
		x[b] = t;
	}
}
